Concepts based on multiple transmit and receive antennas are the most promising techniques for achieving high data rate communication. However, since the carrier frequencies are expected to grow in the future wireless communications systems (e.g. to 5 GHz for so called fourth generation), the path loss also increases and reduces the advantages offered by traditional multiple-input multiple-output (MIMO) system architecture.
To overcome this loss, a new network architecture based on distributed antenna concept has been recently proposed. The key idea of this proposal, referred as distributed MIMO system, comprises distributing groups of antennas over a large geographical area. All the antenna groups are connected to a common base transceiver station (BTS) where the actual radio frequency and base band processing units as well as radio resource and network management functionalities are included. The advantage of this system is that all antenna groups can fully cooperate. However, for a full cooperation, the carrier frequency and phase synchronization is required, and hence, the traditional synchronization methods applied to co-located antennas systems can not be used.
As an example let us consider a case where some antennas belonging to different antenna groups cooperatively create beams into a desired direction. This technique is referred to a distributed beamforming transmission from now on. Clearly this technique requires all active antenna groups to be fully phase synchronized at the carrier level in order to control phase difference between all the transmitted signals. However, the carrier phase synchronization between the antenna groups situated at large distances (e.g. hundred of meters) is a serious problem and is hard to be solved just by calibration of the connection cables between the antenna groups and the common BTS. In the absence of a carrier phase synchronization the cooperative transmission using different antenna groups is limited to non-coherent transmit diversity techniques. This limits most of the gains provided by the possibility to jointly optimize the signals transmitted in different antenna groups at the common BTS. For example, two different unsynchronized antenna groups cannot perform joint beamforming towards a common intended user.
There are methods for carrier phase correction in collocated multiple antenna systems including phase locked loops for synchronizing different oscillators and calibration of the connection cables. These known algorithms relay on the fact that there is only a short distance between the BTS and the antenna groups, i.e., the initial calibration of the connection is well preserved (stable) in time. However, this cannot be applied when there are large distances between BTS and antenna groups. Thus, there is a need to find solutions for carrier phase correction in distributed communication systems using a common BTS.